The invention concerns fast-reacting casting resin on the basis of polyurethanes, processes for their production as well as their use as embedding masses.
The previously known casting resin systems involve not only at normal but also at increased temperatures slower, often hours-long reaction or hardening times. The long reaction periods are limited through slowly proceeding addition reactions of the involved chemical raw materials, and therefore allow only a discontinuous working-up.
It is thus known from DE-OS 28 13 197 to obtain such polyurethanes by reacting an aromatic polyisocyanate with a mixture of castor oil and trimethylol propane into an NCO-group-displaying pre-adduct, and polymerizing the pre-adduct with castor oil or a mixture of castor oil and trimethylol propane. The embedding masses described in this reference distinguish by outstanding characteristics such as high hardness, colorlessness, compatibility with blood, and good working-up characteristics. They are particularly suitable for embedding of membranes such as hollow filaments, tube foils, flat foils and the like. These can be used for construction of separatory arrangements such as dialysators, particularly hemodialysators.
Further such polyurethane embedding masses are known from DE-OS 28 55 243, which discloses as aromatic polyisocyanate 4,4'-diphenylmethane-diisocyanate with a content from 18-22 mol-% dimerised and trimerised diisocyanate, and from DE-OS 29 07 501, in which is described an aromatic polyisocyanate with 10-15 mol-% 3-isocyanatomethyl-3,5,5-trimethyl-cyclohexylisocyanate. These embedding masses distinguish through good storage ability and viscosity characteristics (DE-OS 28 55 243) or through high adhesive strength (DE-OS 29 07 501).
Since now a need exists for such casting resins, which allow hardening within shorter time periods or in continuous processes, whereby particularly the time period up to mold release of the embedding masses is significant, attempts have been made to harden these polyurethanes catalytically.
Thus, e.g. in German patent application P 30 10 030.2-44 embedding masses on the basis of polyurethane compositions from the above mentioned DE-OS 28 13 197, 28 55 243, and 29 07 501 are described, which are hardened with dialkyl tin compounds as catalyst. Through these measures it became possible to shorten the time period up to mold release of the embedded membranes to 20 minutes (with a working-up temperature of 50.degree. C.). Thus, these masses, in connection with a favorable chronological viscosity behavior of the masses during the polymerization, are also suitable for the embedding of membranes in automatic casting machines.
Finally, in DE-OS 24 38 197 there are described polyurethanes which are produced through reaction of an aromatic diisocyanate with a hydroxyl group-containing polymer into a NCO-group-displaying pre-adduct, and polymerization of the pre-adduct with a lower molecular diol in the presence of water and a combination of an organo tin compound and a tertiary amine as catalyst. Therewith both of the catalysts are not added directly as mixture to one of the polymerization components, but are provided initially separated in each one of the components. According to the test examples, which are provided for closer illustration in DE-OS 24 38 197, there results thus with dibutyl tin dilaurate and 1,4-diazabicyclo(2,2,2)-octane in weight ratio 3:1 and a portion of this mixture of 0.2% by weight, relative to the total polyurethane, with a preferred hardening temperature of 50.degree. C., a polyurethane which can be released from the mold after a period of about 1 minute. Involved herewith, though, is a polyurethane foam.
In order to be able to work up polyurethanes, particularly compact embedding masses, more economically in automatic casting machines, casting resins are required which, after the mixing of the NCO-group-displaying pre-adduct ("macroisocyanate"; "hardener") and the polyol ("chain-lengthener") reactant can be removed from the mold within still shorter time periods.